Optical nanoantennas convert freely propagating optical radiation into localized energy in the subwavelength scale, and vice versa. They may be considered as the optical analogues of microwave and radiowave antennas, which are omnipresent in modern society and have a major impact on our daily life. Such optical nanoantennas may have revolutionized the way we wield light. An important characteristic of the optical nanoantenna is the scattering directivity, e.g. in which directions it can efficiently transmit or receive power. Optical antennas may therefore provide light ray redirection into engineered directions at a subwavelength scale. Design of optical nanoantennas with controllable scattering directivity is strongly desirable for a broad range of applications, e.g. in photodetection, fluorescence emission, sensing, colour routing, spectroscopy and so on. Thus, nanoantenna applications may range from advanced photodetection and optical communication to biomedical sensing and near-field microscopy.
A nanoantenna may be considered as a subwavelength bridge between free space optical radiation and localized energy. The localized electromagnetic modes may strongly depend on its geometry and material composition. The material composition and geometry, as well as the surrounding medium, strongly affect the localized electromagnetic modes they support, and consequently determines how optical radiation is received and transmitted. It is greatly desirable to obtain optical nanoantennas with controllable directionality, featuring directed signal reception and transmission with high efficiencies.
It is known in the art to use plasmonic, e.g. metallic, nanoantennas for this purpose. Plasmonic structures have been extensively exploited for nanoantenna design. Such metallic nanoantennas, e.g. a gold V-shaped antenna, a gold Yagi-Uda array antenna or a split-ring resonator, are based on localized surface plasmon resonance (LSPR). However, the intrinsic ohmic losses of metals may be large, e.g. particularly in the visible range, which may impede further efficiency improvement for practical applications. In this type of antenna, the relatively weak magnetic response to the incident light may restrict further engineering of high-performance nanoantennas of this type. Furthermore, since the interaction of metallic nanoantennas with the magnetic component of light may be relatively weak, highly tunable scattering directionality may be difficult to achieve.
Most of the reported directional plasmonic nanoantennas only direct light into one particular engineered direction, and the directivity is difficult to switch into other directions. In addition, intrinsic absorption losses of metals are especially high in the visible range, and the power of the scattered light for a plasmonic nanoantenna is relatively low. Therefore, an opportunity exists for further improvement for practical applications.
Particular plasmonic nanostructures are known in the art which can achieve switched directional scattering into opposite directions, e.g. as disclosed in the European patent application EP 13197308. For example, a double split-ring resonator as known in the art is illustrated in FIG. 1. In such prior-art device, two face-to-face arranged split-ring resonators having different geometries are merged, such that each split-ring resonator part contributes to the light scattering into one direction at a particular wavelength. Thus, directional scattering into opposite directions can be obtained in a device as known in the art. However, the design of such double split-ring resonator structure should be carefully optimized, e.g. the geometric parameters, such as the sizes of the split-ring resonators and the distance and the gap size in between the split-ring resonators, is to be carefully engineered to obtain the desired directivity as function of wavelength.
Another prior-art example is the circular patch nanoantenna with a rod-shaped nanoslit, e.g. a plasmonic nanodisk antenna with a rod-shaped aperture as shown in FIG. 2. In such device, the interference between the resonances of the patch nanoantenna and the nanoslit, also referred to as Fano resonance in the art, results in a dual directional scattering of light into opposite directions at different wavelengths. However, this may have been only demonstrated in the near-infrared range, e.g. near-infrared light in the telecommunication band. The coupling directivity of this prior-art device is shown in detail in FIG. 3.
It has been suggested that all-dielectric nanoantennas, made of high refractive index materials such as silicon, could provide efficient optical manipulation at the nanoscale. For example, it is known in the art that high-index dielectric nanoparticles can exhibit both strong electric and magnetic resonances, which are tunable throughout the visible spectrum, while intrinsic absorption losses may be small due to the low imaginary part of the refractive index. These distinctive properties may enable optimized manipulation of electromagnetic radiation. For example, directional scattering with high forward-to-backward scattering ratio along the propagation direction has been observed for spherical silicon nanoparticles due to the interference between electric and magnetic dipole resonances. Furthermore, rotation of the scattering direction has also been theoretically predicted in asymmetric silicon nanodimers.